Optical Pulse Time Spreader and Optical Code Division Multiplexing Transmission Device

ABSTRACT

The ratio P/W between the peak value P and the subpeak value W of the autocorrelation waveform, and the ratio P/C between the peak value P of the autocorrelation waveform and the maximum peak value C of the cross correlation waveform are both large.  
     The present invention comprises phase control means of a structure in which an SSFBG  40  having fifteen unit FBGs arranged in series in the waveguide direction is fixed to the core of the optical fiber  36  that comprises the core  34  and cladding  32 . The difference Δn between the maximum and minimum of the effective refractive index of the optical fiber is 6.2×10 −5 . The phase difference of Bragg reflected light from two unit diffraction gratings that adjoin one another from front to back and provide equal code values is given by 2πM+(π/2) where M is an integer. Further, the phase difference of the Bragg reflected light from two unit diffraction gratings that adjoin one another from front to back and provide different code values is given by 2πM+(2N+1)π+(π/2) where M and N are integers.

TECHNICAL FIELD

The present invention relates to an optical pulse time spreader thattime-spreads an optical pulse into chip pulses. More specifically, thepresent invention relates to an optical encoder or optical decoder thatis an applied example of an optical pulse time spreader. Moreparticularly, the present invention relates to an optical encoder oroptical decoder comprising phase control means of a constitution inwhich unit diffraction gratings are arranged in series along thewaveguide direction of the optical fiber. Further, the present inventionrelates to an optical code division multiplexing transmission methodthat is implemented by using the optical pulse time spreader and adevice for implementing this method.

BACKGROUND ART

In recent years, the demand for communications has increased rapidly asa result of the spread of the Internet and so forth. High capacitynetworks have accordingly been completed at high speed by using opticalfiber. Further, in order to establish high-capacity communications, anoptical multiplexing technology that transmits a plurality of channels'worth of optical pulse signals together via one optical fibertransmission line has been investigated.

As optical multiplexing technology, optical time division multiplexing(OTDM), wavelength division multiplexing (WDM) and optical code divisionmultiplexing (OCDM) have been intensively researched. Among thesetechnologies, OCDM has the merit of flexibility on the operation side,that is, of having no restrictions on the time axis allocated one bit ata time of the optical pulse signals that are transmitted and received inOTDM and WDM and so forth. Further, OCDM has the merit that a pluralityof channels can be established in the same time slot on the time axis ora plurality of communication channels can also be established with thesame wavelength on the wavelength axis.

Subsequently, the expression ‘optical pulse signal’ signifies an opticalpulse train reflecting a binary digital signal. That is, an opticalpulse train reflecting a binary digital signal in correspondence withthe existence and nonexistence of optical pulses constituting theoptical pulse train on a time axis with respect to an optical pulsetrain in which optical pulses stand in a row at regular fixed intervals(time interval corresponding to the reciprocal of the frequencycorresponding to the bit rate) is an optical pulse signal.

OCDM is a communication method that extracts signals by means of patternmatching by allocating codes (patterns) that are different for eachchannel. That is, OCDM is an optical multiplexing technology thatencodes an optical pulse signal by means of an optical code that isdifferent for each communication channel on the transmission side andwhich restores the original optical pulse signal by performing decodingby using the same optical codes on the reception side as on thetransmission side.

Because only the optical pulse signals whose codes correspond areextracted and processed as effective signals during decoding, an opticalpulse signal that consists of light rendered by combining the samewavelengths or a plurality of wavelengths can be allocated to aplurality of communication channels. Further, because a passive lightelement such as a Fiber Bragg Grating (FBG) can be used as the phasecontrol means of the optical encoder to perform the phase controlrequired for code processing, it is possible to deal with a highercommunication rate without the encoding processing being subject toelectrical restrictions. Further, suppose that a plurality of channelscan be multiplexed at the same time and same wavelength andlarge-capacity data communications are possible. In comparison with OTDMand WDM and so forth, the focus is on being able to rapidly increase thecommunication capacity.

As OCDM encoding means, an optical phase code system that uses the phaseof light as code is known. More specifically, a Superstructured FiberBragg Grating (SSFBG) is used as the encoder and decoder (See Non-patentdocuments 1 and 2, or Patent document 1, for example).

The operating principles in a case where an optical pulse time spreadercomprising phase control means formed by using an SSFBG encoder is usedas an encoder and decoder will now be described with reference to FIGS.1(A) to 1(E). FIG. 1(A) shows the time waveform of an input opticalpulse. FIG. 1(E) serves to illustrate an aspect in which an encodedoptical pulse train that has been encoded by an encoder is decoded by adecoder.

The input optical pulse shown in FIG. 1(A) is encoded as a result ofbeing input to an encoder 10 from an optical fiber 12 via an opticalcirculator 14. The input optical pulse then passes through the opticalfiber 18 via the optical circulator 14 once again before being input toa decoder 20 via an optical circulator 22. Further, an autocorrelationwaveform is generated as a result of decoding by a decoder 20 and theautocorrelation waveform passes through an optical fiber 26 via theoptical circulator 22.

The encoder 10 and decoder 20 shown in FIG. 1(E) are an SSFBGconstituted by arranging four unit Fiber Bragg Gratings (FBG) in thewaveguide direction of the optical fiber. Here, as an example, thefunctions of the encoder 10 and decoder 20 will be described by using afour-bit optical code (0, 0, 1, 0). Here, the number of items in thenumerical sequence consisting of ‘0’s and ‘1’s that provides the opticalcode is also called the codelength. In this example, the codelength is4. Further, the numerical sequence providing the optical code is calleda code string and each item ‘0’ and ‘1’ of the code string is also knownas a chip. Further, the values 0 and 1 are also called code values.

The unit FBGs 10 a, 10 b, 10 c, and 10 d constituting the encoder 10correspond with a first chip ‘0’ of the abovementioned optical codes, asecond chip ‘0’, a third chip ‘1’, and a fourth chip ‘0’ respectively.The determination of whether the code value is 0 or 1 is the phaserelationship of the Bragg reflected light that is reflected by adjacentFBG units. That is, because the first chip and second chip have an equalcode value 0, the phase of the Bragg reflected light reflected by unitFBG 10 a corresponding with the first chip and the phase of the Braggreflected light reflected by unit FBG 10 b corresponding with the secondchip are equal. Further, because the code value of the second chip is 0and the code value of the third chip is 1, the two chips have mutuallydifferent values. Therefore, the difference between the phase of theBragg reflected light reflected by unit FBG 10 b corresponding with thesecond chip and the phase of the Bragg reflected light reflected by unitFBG 10 c corresponding with the third chip is n. Likewise, because thecode value of the third chip is 1 and the code value of the fourth chipis 0, the two chips have mutually different values. Therefore, thedifference between the phase of the Bragg reflected light reflected byunit FBG 10 c corresponding with the third chip and the phase of theBragg reflected light reflected by unit FBG 10 d corresponding with thefourth chip is π.

Thus, because the phases of the Bragg reflected light from the unit FBGsare changed, the specified optical code is also known as ‘optical phasecode’.

A process in which an autocorrelation waveform is formed as a result ofan optical pulse being encoded by an encoder and converted to an encodedoptical pulse train and the encoded optical pulse train being decoded bya decoder will be described next. When the single optical pulse shown inFIG. 1(A) is input from the optical fiber 12 to the encoder 10 via theoptical circulator 14 and optical fiber 16, Bragg reflected light fromthe unit FBGs 10 a, 10 b, 10 c, and 10 d is generated. Therefore,suppose that the Bragg reflected light from the unit FBGs 10 a, 10 b, 10c, and 10 d is a, b, c, and d. That is, the single optical pulse shownin FIG. 1(A) is converted into an encoded optical pulse train as aresult of time spreading of the Bragg reflected light a, b, c, and d.

When the Bragg reflected light a, b, c, and d is represented on a timeaxis, an optical pulse train resulting from arrangement at specifiedintervals that depend on the method of arranging the unit FBGs 10 a, 10b, 10 c, and 10 d on the time axis through division into four opticalpulses is constituted as shown in FIG. 1(B). Therefore, an encodedoptical pulse train is an optical pulse train that is produced as aresult of time-spreading an optical pulse that is input to the encoderas a plurality of optical pulses on a time axis.

FIG. 1(B) shows an encoded optical pulse train that passes through theoptical fiber 18 with respect to the time axis. In FIG. 1(B), for thepurpose of a quick representation of the encoded optical pulse train,the optical pulses are shown displaced in the vertical axis direction.

The Bragg reflected light of unit FBG 10 a is the optical pulse denotedby a in FIG. 1(B). Likewise, the Bragg reflected light of FBG 10 b, FBG10 c, and FBG 10 d are optical pulses denoted by b, c, d respectively inFIG. 1(B). The optical pulse denoted by a is an optical pulse that isreflected by the unit FBG 10 a closest to the input end of the encoder10 and is therefore in the most temporally advanced position. Theoptical pulses denoted by b, c, and d are each Bragg reflected lightfrom the FBG 10 b, FBG 10 c, and FBG 10 d respectively. Further, the FBG10 b, FBG 10 c, and FBG 10 d stand in a line in a row from the input endof the encoder 10 and, therefore, the optical pulses denoted by b, c,and d stand in a line in the order b, c, d after the optical pulsedenoted by a as shown by FIG. 1(B). In the subsequent description, theoptical pulses corresponding with the Bragg reflected light a, Braggreflected light b, Bragg reflected light c, and Bragg reflected light drespectively are also represented as the optical pulse a, optical pulseb, optical pulse c, and optical pulse d. Further, the optical pulse a,optical pulse b, optical pulse c, and optical pulse d are also eachcalled chip pulses.

The relationship between the phases of the Bragg reflected light a, b,c, and d that constitute the encoded optical pulse train is as followsas mentioned earlier. The phase of the Bragg reflected light a and thephase of the Bragg reflected light b are equal. The difference betweenthe phase of the Bragg reflected light b and the phase of the Braggreflected light c is n. The difference between the phase of the Braggreflected light c and the phase of the Bragg reflected light d is n.That is, when the phase of the Bragg reflected light a is taken as thereference, the phases of the Bragg reflected light a, Bragg reflectedlight b, and Bragg reflected light d are equal and the phase of theBragg reflected light c differs by n from the phases of the Braggreflected light a, Bragg reflected light b, and Bragg reflected light d.

Therefore, in FIG. 1(B), the optical pulses corresponding with the Braggreflected light a, the Bragg reflected light b and Bragg reflected lightd are denoted by solid lines and the optical pulse corresponding withthe Bragg reflected light c is denoted by a dotted line. That is, inorder to distinguish the relationship between the phases of therespective Bragg reflected light, solid lines and dotted lines are usedto represent the corresponding optical pulses. The phases of the opticalpulses denoted by a solid line are in a mutually equal relationship andthe phases of optical pulses denoted by dotted lines are in a mutuallyequal relationship. Further, the phases of the optical pulses denoted bya solid line and the optical pulses denoted by a dotted line differ by nfrom one another.

An encoded optical pulse train is input to the decoder 20 via theoptical circulator 22 after passing through the optical fiber 18.Although the decoder 20 has the same structure as the encoder 10, theinput end and output end are reversed. That is, the unit FBGs 20 a, 20b, 20 c, and 20 d stand in a line in order starting from the input endof the decoder 20 but the unit FBG 20 a and unit FBG 10 d correspond.Further, a unit FBG 20 b, unit FBG 20 c and unit FBG 20 d likewisecorrespond with the unit FBG 10 c, unit FBG 10 b, and unit FBG 10 arespectively.

In the encoded optical pulse train that is input to the decoder 20, theoptical pulse a constituting the encoded optical pulse train is firstBragg-reflected by the unit FBGs 20 a, 20 b, 20 c, and 20 d. This aspectwill be described with reference to FIG. 1(C). In FIG. 1(C), thehorizontal axis is the time axis. Further, the relationship before andafter a time is illustrated by expediently assigning 1 to 7, wheresmaller numerical values denote increasingly early times.

FIG. 1(C) shows an encoded optical pulse train with respect to the timeaxis in the same way as FIG. 1B. When the encoded optical pulse train isinput to the decoder 20, the encoded optical pulse train is firstBragg-reflected by unit FBG 20 a. The reflected light that isBragg-reflected by unit FBG 20 a is shown as ‘Bragg reflected light a’.Likewise, the reflected light that is Bragg-reflected by the unit FBG 20b, unit FBG 20 c, and unit FBG 20 d is shown as the Bragg reflectedlight b′, c′, and d′ respectively.

The optical pulses a, b, c and d constituting the encoded optical pulsetrain are Bragg-reflected by unit FBG 20 a and stand in a line on thetime axis of the string denoted by a′ in FIG. 1(C). The optical pulse athat is Bragg-reflected by unit FBG 20 a is an optical pulse that has apeak in a certain position that is denoted by 1 on the time axis. Theoptical pulse b that is Bragg-reflected by unit FBG 20 a is an opticalpulse with a peak in a certain position that is denoted by 2 on the timeaxis. Likewise, the optical pulse c and optical pulse d are opticalpulses with a peak in a certain position denoted by 3 and 4 respectivelyon the time axis.

The optical pulses a, b, c, and d that constitute the encoded opticalpulse train are also Bragg-reflected by unit FBG 20 b and stand in aline on the time axis of the string denoted by b′ in FIG. 1(C). TheBragg-reflected reflected light b′ that is reflected by unit FBG 20 bhas a phase that is shifted by n in comparison with the phases of theBragg-reflected light a′, c′ and d′. Therefore, the string of opticalpulses that stand in a line on the time axis of the string denoted by a′and the string of optical pulses that stand in a line on the time axisof the string denoted by b′ have phases that are all shifted by π.

As a result, whereas a string of optical pulses that stand in a line inthe order 1 to 4 on the time axis denoted by a′ stand in a line in theorder of a solid line, solid line, dotted line, and solid line, a stringof optical pulses that stand in a line in the order 2 to 5 on the timeaxis denoted by b′ stand in a line in the order of a dotted line, dottedline, solid line, and dotted line. The displacement on the time axis ofthe optical pulse train denoted by a′ and the optical pulse traindenoted by b′ is because, among the optical pulses constituting theencoded optical pulse train, the optical pulse a is input to the decoder20 before the optical pulse b.

Likewise, the optical pulses a, b, c, and d that constitute the encodedoptical pulse train are also Bragg-reflected by the unit FBG 20 c andunit FBG 20 d and the optical pulses stand in a line on the time axis ofthe strings denoted by c′ and d′ respectively in FIG. 1(C). TheBragg-reflected light c′ and d′ reflected by the unit FBG 20 c and unitFBG 20 d have phases that are equal in comparison with theBragg-reflected light a′. Therefore, in FIG. 1(C), the optical pulsetrain denoted by c′ and the optical pulse train denoted by d′ stand in aline on the time axis. The optical pulses related to the Bragg-reflectedlight a′, c′, and d′ are shifted in parallel on the time axis but themutual phase relationship between the optical pulses related to theBragg-reflected light is the same.

FIG. 1(D) shows the autocorrelation waveform of the input optical pulsesthat are decoded by the decoder 20. The horizontal axis is the time axisand corresponds to the illustration shown in FIG. 1(C). Theautocorrelation waveform is obtained by the sum of the Bragg-reflectedlight a′, b′, c′, and d′ from the respective unit FBGs of the decoderand, therefore, all the Bragg-reflected light a′, b′, c′ and d′ shown inFIG. 1(C) is brought together. Because the optical pulses related to theBragg-reflected light a′, b′, c′ and d′ are all added together with thesame phase at the time shown as 4 on the time axis of FIG. 1(C), amaximum peak is formed. Further, because two optical pulses denoted by adotted line and one optical pulse denoted by a solid line are addedtogether at the time shown as 3 on the time axis of FIG. 1(C), oneoptical pulse's worth of peaks whose phases differ by n are formed forthe maximum peak at the time shown as 4. Further, because two opticalpulses denoted by a solid line and one optical pulse denoted by a dottedline are added together at the time shown as 1 on the time axis of FIG.1(C), one optical pulse's worth of peaks whose phases are equal areformed for the maximum peak at the time shown as 4.

As described hereinabove, the optical pulses are encoded by the encoder10 to produce an encoded optical pulse train and the encoded opticalpulse train is decoded by the decoder 20 to generate an autocorrelationwaveform. In the example taken here, an optical code (0,0,1,0) of fourbits (codelength 4) is used but the description above is equally valideven in cases where optical code is not used.

The schematic structure of conventional phase control means will now bedescribed with reference to FIGS. 2(A) and 2(B). FIG. 2(A) is aschematic cross-sectional view of the phase control means. The phasecontrol means has a structure in which an SSFBG 30 is fixed to a core 34of an optical fiber 36 comprising the core 34 and cladding 32. The SSFBG30 is constituted such that 15 unit FBGs are arranged in series in thewaveguide direction of the core 34 constituting the optical waveguide ofthe optical fiber 36.

When the optical phase code which is set for the phase control means ofthe conventional optical pulse time spreader shown in FIG. 2(A) iswritten as a 15-bit code string, the result is(0,0,0,1,1,1,1,0,1,0,1,1,0,0,1). Further, the relationship ofcorrespondence between the abovementioned optical code and the unit FBGsarranged in series in the core 34 is as follows. That is, the unit FBGs,which are arranged in a direction extending from the left end to theright end of the SSFBG 30 shown in FIG. 2(A) and the chips, which arearranged in a direction extending from the left end to the right end of(0,0,0,1,1,1,1,0,1,0,1,1,0,0,1) that represents the optical codes of theunit FBGs noted as the abovementioned 15-bit code string correspond withone another one-on-one.

FIG. 2(B) schematically shows the refractive index modulation structureof the SSFBG 30 shown in FIG. 2(A). The horizontal axis is a positioncoordinate in the longitudinal direction of the optical fiber 36 formingthe SSFBG 30. The vertical axis represents the refractive indexmodulation structure of the optical fiber 36 and the difference betweenthe maximum and minimum of the effective refractive index of the opticalfiber 36 is represented as Δn. Further, in FIG. 2(B), the refractiveindex modulation structure of the optical fiber 36 is drawn partiallyenlarged.

The refractive index modulation cycle is Λ. Therefore, the Braggreflection wavelength λ is given by λ=2N_(eff)Λ. Here, N_(eff) is theeffective refractive index of the optical fiber 36. In the subsequentdescription, the effective refractive index is also simply called therefractive index for the sake of simplification.

In FIG. 2(A), when the phases of the Bragg-reflected light of adjacentunit FBGs differ by n, the intervals between adjacent unit FBGs areshown shaded black. Further, when the phases of the Bragg reflectedlight of adjacent unit FBGs are equal, an optical modulation structurein which the intervals between the unit FBGs are continuous is shown. Onthe other hand, in FIG. 2(B), when the phases of the Bragg-reflectedlight of adjacent unit FBGs differ by π, black triangles are shown addedto the intervals of the two unit FBGs.

When the phases of the Bragg reflected light of adjacent unit FBGs areequal, the refractive index modulation structure of the two unit FBGs isa continuous cycle structure. On the other hand, when the phases of theBragg reflected light of adjacent unit FBGs differ by π, the refractiveindex modulation structure of the two unit FBGs have a shift of only π(jump in the pie phase) inserted at the boundary between the two unitFBGs.

Table 1 shows the relationship between the optical phase code (0, 0, 0,1, 1, 1, 1, 0, 1, 0, 1, 1, 0, 0, 1) and the phase difference of theBragg reflected light of adjacent unit FBGs for implementing the opticalcode. At the top of Table 1, the code values of the optical phase codeestablished for the conventional phase control means shown in FIG. 2(A)are shown lined up in a row as code. Further, the phase difference ofthe Bragg reflected light of adjacent unit FBGs is shown as the phaseshift amount in the bottom level of Table 1. The unit FBGs arrangedextending from the left end to the right end of the SSFBG 30 shown inFIG. 2(A) and the chips arranged extending from the left end to theright in brackets representing the optical phase code (0, 0, 0, 1, 1, 1,1, 0, 1, 0, 1, 1, 0, 0, 1) correspond one for one. TABLE 1

The geometric interval between adjacent unit FBGs in which the phaseshift amount is n is converted to a phase value to become π/2 due tolight traveling there and back between adjacent unit FBGs. Generally,when the interval between adjacent unit FBGs for which the phase shiftamount is n is converted to a phase value, the interval is given byπN+(π/2) with N as an integer. That is, the phase difference of theBragg reflected light from adjacent unit FBGs for which the phase shiftamount is n is given by 2πN+π. Further, the geometric interval betweenadjacent unit FBGs for which the phase shift amount is 0 is converted toa phase value and given by πN, and the phase difference of the Braggreflected light from the two unit FBGs is given by 2πN.

Further, subsequently, when the phase shift amount is written, generalnotation such as πN+(π/2) is sometimes omitted and also written simplyas π/2.

[Non-Patent Document 1]

Akihiko Nishiki, Hisashi Iwamura, Hideyuki Kobayashi, Satoko Kutsuzawa,Saeko Oshiba ‘Development of OCDM phase encoder using SSFBG’ TechnicalReport of IEICE. OFT2002-66, (2002-11),

[Non-Patent Document 2]

Hideyuki Sotobayashi, ‘Optical code division multiplexing network’,Applied Physics, Volume 71, 7. (2002) pages 853 to 859,

[Patent Document 1]

U.S. Pat. No. 6,628,864,

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

However, when encoding and decoding are executed by means of an opticalpulse time spreader that comprises conventional phase control meansexemplified by FIGS. 2(A) and 2(B), there is interference as a result ofthe feet of the chip pulses constituting the encoded optical pulse trainoverlapping on the time axis and, therefore, the encoding and decodingillustrated with reference to FIG. 1 are sometimes not implementedaccording to the design. The above problem in a case where encoding anddecoding are executed by an optical pulse time spreader comprisingconventional phase control means will be described specifically withrespect to the effect of interference between the Bragg reflected lightfrom the unit FBGs constituting the phase control means, for example,with reference to FIGS. 3(A) to 3(F).

FIG. 3(F) is a conceptual constitutional cross-sectional view of thephase control means 38 of an optical pulse time spreader comprising aunit FBG41 and unit FBG42. When the input optical pulse 44 is input tothe phase control means 38, the respective Bragg reflected light 46 andBragg reflected light 48 are generated by the unit FBGs 41 and 42.

FIG. 3(A) shows a time waveform for an optical pulse that is input tothe phase control means 38. FIG. 3(B) is a time waveform of the Braggreflected light 46 and Bragg reflected light 48 that are reflected bythe unit FBGs 41 and 42. FIG. 3(B) shows a chip pulse 46P (dotted line)and a chip pulse 48P (solid line) that correspond with the Braggreflected light 46 and Bragg reflected light 48 respectively. As shownin FIG. 3(B), the feet of the chip pulse 46P and chip pulse 48P overlaptemporally as indicated by the oblique lines.

When the phase difference between the chip pulse 46P and chip pulse 48Pis π, the encoded optical pulse train is as shown in FIG. 3(C). That is,as shown in FIG. 3(B), in the part where the chip pulse 46P and chippulse 48P temporally overlap (the part shaded with the oblique lines inFIG. 3(B)), the chip pulse 46P and chip pulse 48P interfere with oneanother and cancel each other out. Hence, the optical intensity growssmaller in the interval on the time axis between the chip pulse 46P andchip pulse 48P.

On the other hand, when the phase difference between the chip pulse 46Pand chip pulse 48P is 0, the encoded optical pulse train is as shown inFIG. 3(D). That is, as shown in FIG. 3(B), in the part where the chippulse 46P and chip pulse 48P temporally overlap (the part shaded withthe oblique lines in FIG. 3(B)), the chip pulse 46P and chip pulse 48Pinterfere with one another and reinforce each other. Hence, the opticalintensity is large in comparison with the case shown in FIG. 3(C) in theinterval on the time axis between the chip pulse 46P and chip pulse 48P.

FIG. 3(E) shows an encoded optical pulse train in a case where the phasedifference between the chip pulses 46P and 48P is π/2 or 3π/2. In thiscase, the optical intensity between chip pulse 46P and chip pulse 48P onthe time axis is larger than the case shown in FIG. 3(C) and smallerthan the case shown in FIG. 3(D).

In order to implement a binary code by means of an SSFBG, the phasedifference between adjacent unit FBGs can also be implemented throughdetermination as two types π/2 and 3π/2 instead of 0 and π. Thus, if anoptical encoder in which an SSFBG constituted by agreeing upon the phasedifference is the phase control means is used, the encoded optical pulsegenerated by the optical encoder is such that the intensities betweenchip pulses as shown in FIG. 3(E) are equal even when the phasedifference between adjacent unit FBGs is any of π/2 and 3π/2. Theadvantages obtained as a result of the intensities between the chippulses constituting the encoded optical pulse all being equal will bedescribed in detail subsequently.

A case where encoding and decoding are executed by a conventionaloptical pulse time spreader exemplified by FIGS. 2(A) and 2(B) wasinvestigated hereinabove while considering the description whilereferencing FIGS. 3(C) to 3(E).

An encoded waveform, autocorrelation waveform, and cross correlationwaveform in a case where encoding and decoding are executed by theconventional optical pulse time spreader exemplified by FIGS. 2(A) and2(B) are shown in FIGS. 4(A) to 4(C). In FIGS. 4(A) to 4(C), thehorizontal axis shows time calibrated in ps units and the vertical axisis shown calibrated with the optical intensity on an optional scale.Further, in the drawings, which show encoded waveforms representing theencoded optical pulse train shown in FIG. 4(A), the area ratio isillustrated as 0.15 but the area ratio has the following meaning. Thatis, the area ratio is the ratio between the energy of the optical pulsethat is input to a conventional optical pulse time spreader and theenergy of an encoded optical pulse train. The energy of the opticalpulse that is input to the conventional optical pulse time spreader isin proportion to the time axis and the area circled by a curved linethat provides the intensity distribution of the optical pulse, in thedrawing (not shown) that represents the time waveform. On the otherhand, the energy of the encoded optical pulse train is in proportion tothe time axis and the area circled by the curved line that provides theintensity distribution of the encoded waveform in the drawing shown inFIG. 4(A).

When the encoded waveform shown in FIG. 4(A) is viewed, the intensity ofthe chip pulses constituting the encoded optical pulse train is small inpart Q where code values emerge as being mutually different. Further,the intensity of the chip pulses constituting the encoded optical pulsetrain increases in parts R and S in which the same code values appearconsecutively.

Here, part Q, where code values emerge as being mutually different,corresponds with a point where the code values constituting the opticalcode appear as ( . . . ,0,1,0,1, . . . ). Further, parts R and S, wherethe same code values appear consecutively, each correspond with pointswhere the code values constituting the optical code appear as ( . . .,1,1,1,1, . . . ) and (,0,0,0,0, . . . ).

That is, large variations in the intensity of the encoded optical pulsetrain occur depending on the order of arrangement of the code valuesconstituting the optical code. As a result of such variations, the peakvalue of the autocorrelation waveform obtained as a result of decodingis small and leads to the occurrence of an obstacle in the step ofextracting the autocorrelation waveform by removing the crosscorrelation waveform from the decoded signal.

Here, the effect on the autocorrelation waveform obtained throughdecoding as a result of the occurrence of variations in the intensity ofthe encoded optical pulse train due to the abovementioned interferencebetween the chip pulses constituting the encoded optical pulse trainwill be described with reference to FIGS. 5(A) and 5(B).

FIG. 5(A) shows an autocorrelation waveform with the same optical code(0,0,0,1,1,1,1,1,0,1,0,1,1,0,0,1) as the optical code set for the phasecontrol means shown in FIGS. 2(A) and 2(B). FIG. 5(B) shows the crosscorrelation waveform. The horizontal axis in each of FIGS. 5(A) and 5(B)is a time axis shown calibrated in ps units and the vertical axis isshown calibrated with the optical intensity on an optional scale.

The autocorrelation waveform and cross correlation waveform shown inFIGS. 5(A) and 5(B) are obtained by way of a simulation in whichvariations in the intensity of the encoded optical pulse train can beignored because the width of the chip pulses constituting the encodedoptical pulse train is relatively narrow in comparison with the timeinterval between the chip pulses and the earlier mentioned interferencebetween the chip pulses does not exist. The cross correlation waveformis obtained by using completely inverted optical codes to decode thecode values of the optical codes. That is, the optical code for thedecoding that is used in order to simulate the cross correlationwaveform is (1,1,1,0,0,0,0,1,0,1,0,0,1,1,0).

The relative value of the peak value of the autocorrelation waveformshown in FIG. 5(A) (also subsequently referred to as the ‘signal peak’and indicated by ‘P’) is 225 and the relative value of the largestsubpeak (subsequently represented by ‘W’) of the subpeak that exists onboth sides of the signal peak is 9. Therefore, the ratio between thesignal peak value P and the subpeak value W for the largest subpeak is25 (P/W=225/9=25). Further, the maximum peak value (represented by Csubsequently) of the cross correlation waveform shown in FIG. 5(B) is 49and the ratio between the signal peak value P and the maximum peak valueC of the cross correlation waveform is 4.6(P/C=225/49≈4.6).

When P/W is likewise found for the autocorrelation waveform shown inFIG. 4(B) that is obtained when encoding and decoding are executed bymeans of the conventional phase control means exemplified by FIGS. 2(A)and 2(B), the results are as follows. Each time an autocorrelationwaveform is found, an experiment was performed by reversing the order ofarrangement of the code values of the encoder and decoder. BecauseP=7.36 and W=0.624, P/W=7.36/0.624≈11.8. If the result of the simulationin the ideal case shown in FIG. 5(A) is P/W=25, P/W is substantiallyhalf the size when compared with the results of the simulation.

Furthermore, when P/C is likewise found for the cross correlationwaveform shown in FIG. 4(C) that is obtained when encoding and decodingare executed by means of the conventional phase control meansexemplified by FIGS. 2(A) and 2(B), the results are as follows. Eachtime a cross correlation waveform is found, an experiment was performedby making the order of arrangement of the code values of the encoder anddecoder the same. Because P=7.36 and C=2.73, P/W=7.36/2.73≈2.7. If theresult of the simulation in the ideal case shown in FIG. 5(B) isP/C=4.6, P/C is on the order of 60% of the size when compared with theresults of the simulation.

It can be seen that, with respect to the autocorrelation waveform andcross correlation waveform obtained when encoding and decoding areexecuted by conventional phase control means, both P/W and P/C havesmall values in comparison with the results of a simulation in an idealcase, as mentioned earlier. Incidentally, as P/W and P/C both increase,it is easy to separate the autocorrelation waveform, which is a signalin a decoded case.

When optical code division multiplexing communication is performed byusing an optical pulse time spreader that comprises conventional phasecontrol means for which the values of P/W and P/Care of the magnitudeshown in FIGS. 4(B) and 4(C), the following problems arise. That is,because the intensity of the encoded optical pulse train decreases as aresult of the absorption of light by the optical fiber constituting thetransmission line and of the invasion of the encoded optical pulse trainby optical noise that is produced by an optical amplifier that isintegrated into the device if required, the extraction of theautocorrelation waveform peak is problematic.

Furthermore, when there is a separation from the characteristic resultsof the code correlation simulation for which an ideal case is assumed,the correlation characteristic of the optical pulse time spreader cannotbe estimated and an OCDM or other system design that uses an opticalpulse time spreader as an encoder and decoder is problematic.

Therefore, an object of the present invention is to provide an opticalpulse time spreader that approximates P/W and P/C when there is nointerference between chip pulses and large values of P/W and P/C areobtained. That is, an object of the present invention is to provide anoptical pulse time spreader that separates the cross correlationwaveform component from the decoded optical pulse signal from theautocorrelation waveform and obtains larger values of P/Wand P/C themore the identification conditions set for the judgment circuit foridentifying the autocorrelation waveform can be relaxed.

A further object is to provide an optical code division multiplexingmethod that uses the optical pulse time spreader of the presentinvention and an optical code division multiplexing transmission devicefor implementing this method. As a result, the design of the opticalcode division multiplexing transmission device can be simplified.

MEANS FOR SOLVING THE PROBLEMS

The optical pulse time spreader constituting the first invention has afunction that time-spreads an optical pulse as a series of chip pulsestream that are sequentially arranged on the time axis by means ofencoding that employs optical phase code and outputs the series of chippulse stream, and possesses the following characteristics.

That is, the optical pulse time spreader comprises phase control meansthat generate the series of chip pulse stream by providing a phasedifference between adjacent chip pulses among chip pulses correspondingwith the code values constituting the optical phase code. Further, whenthe adjacent code values are equal, the phase control means give thephase difference between the corresponding chip pulses by2πM+(π/2)  (1),and, when the adjacent code values are different, the phase controlmeans give the phase difference between the corresponding chip pulses by2πM+(2N+1)π+(π/2)  (2), where M and N are integers.

Alternatively, the phase control means has a function for giving thephase difference between chip pulses corresponding with differentadjacent code values by means of Equation (1) above and for giving thephase difference between chip pulses corresponding with adjacent equalcode values by means of Equation (2).

Further, a constitution in which unit diffraction gratings, which arearranged in a row and correspond one for one with the code values thatconstitute the optical code, are disposed in series in the waveguidedirection of the optical waveguide is used for the phase control meanswhich is preferably constituted so that the phase difference of theBragg reflected light from two adjacent diffraction gratings in thevicinity satisfies Equations (1) and (2) above. The Bragg reflectedlight corresponds to the above chip pulses. That is, the chip pulsesoutput by the phase control means constituted by the unit diffractiongratings are constituted by Bragg reflected light.

Further, when a constitution in which the unit diffraction gratings aredisposed in series in the waveguide direction of the optical waveguideis used for the phase control means, the refractive index modulationintensity of the periodic refractive index modulation structure of theunit diffraction grating is preferably constituted by performing apodization by means of a window function.

Further, a constitution whereby the refractive index modulationintensity of the periodic refractive index modulation structure in whichthe unit diffraction gratings arranged in series in the waveguidedirection of the optical waveguide are formed is monotonously increasedin the waveguide direction of the optical waveguide is suitable.

In addition, more specifically, the optical pulse time spreader issuitably constituted comprising: phase control means comprising a numberJ (J is a natural number of 2 or more) of unit diffraction gratings,wherein numbers from first to Jth are assigned to the unit diffractiongratings sequentially from one end of the optical waveguide to the otherend thereof; and the reflectance R_(i) from the ith (2≦i≦J) unitdiffraction grating is given by reflectance R_(i)=R_(i)−1/(1−R_(i-1))²(3).

Further, the optical code division multiplexing transmission deviceconstituting the second invention is characterized by using the opticalpulse time spreader of the first invention as the encoder and decoder.

EFFECTS OF THE INVENTION

The optical pulse time spreader constituting the first invention givesthe phase difference between the chip pulses corresponding with the codevalues by means of Equations (1) and (2) above. Therefore, as alreadydescribed schematically with reference to FIG. 3(E), the intensitybetween the chip pulses constituting the series of chip pulse streamgenerated by the optical pulse time spreader of the first invention isequal in either of the cases where the phase difference is given byEquations (1) and (2) above.

As a result, P/W and P/C are approximated when there is no interferencebetween the chip pulses. Further, large values are obtained for P/W andP/C in comparison with a conventional optical encoder or an opticalpulse time spreader that has been used as an optical decoder.

Further, when a constitution in which unit diffraction gratings that arearranged in a row and correspond one for one with the code valuesconstituting the optical code is used for the phase control means, byconstituting the refractive index modulation intensity of the periodicrefractive index modulation structure of the unit diffraction gratingsuch that same increases monotonously in the waveguide direction of theoptical waveguide, the Bragg reflected light intensity from the unit FBGdisposed in a position close to the entry end of the optical waveguideand the Bragg reflected light intensity from the unit FBG disposed in aposition spaced apart from the entry end of the optical waveguide can beequalized.

Furthermore, because the optical pulse time spreader of the firstinvention is used as an encoder and decoder, the optical code divisionmultiplexing transmission device of the second invention obtains largevalues for both the P/Wand P/C in comparison with a conventional opticalpulse time spreader. Therefore, even when the intensity of the encodedoptical pulse train decreases while same is propagated by the opticaltransmission line or optical noise invades the optical transmissionline, the autocorrelation waveform peak can be extracted highly reliablythrough decoding.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 serves to illustrate the operating principles of an encoder anddecoder;

FIG. 2 is a schematic explanatory diagram of the refractive indexmodulation structure of the conventional phase control means;

FIG. 3 serves to illustrate the effect of interference of the Braggreflected light from unit FBGs;

FIG. 4 shows an encoded waveform, autocorrelation waveform and crosscorrelation waveform of a conventional optical pulse time spreader;

FIG. 5 shows the autocorrelation waveform and cross correlation waveformof optical code (0,0,0,1,1,1,1,0,1,0,1,1,0,0,1);

FIG. 6 is a schematic explanatory diagram of the refractive indexmodulation structure of the phase control means of the optical pulsetime spreader of a first embodiment;

FIG. 7 is a schematic constitutional view of the characteristicevaluation device of the optical pulse time spreader;

FIG. 8 shows an encoded waveform, autocorrelation waveform and crosscorrelation waveform of the optical pulse time spreader of the firstembodiment;

FIG. 9 is a schematic explanatory diagram of the refractive indexmodulation structure of the phase control means of the optical pulsetime spreader of a second embodiment;

FIG. 10 shows an encoded waveform, autocorrelation waveform and crosscorrelation waveform of the optical pulse time spreader of the secondembodiment;

FIG. 11 shows an encoded waveform, autocorrelation waveform and crosscorrelation waveform of a conventional optical pulse time spreader;

FIG. 12 is a schematic explanatory diagram of the refractive indexmodulation structure of the phase control means of the optical pulsetime spreader of a third embodiment;

FIG. 13 shows an encoded waveform, autocorrelation waveform and crosscorrelation waveform of an optical pulse time spreader of the thirdembodiment;

FIG. 14 shows an encoded waveform, autocorrelation waveform and crosscorrelation waveform of a conventional optical pulse time spreader; and

FIG. 15 is a schematic block constitutional view of the OCDMtransmission device.

EXPLANATIONS OF LETTERS OR NUMERALS

-   -   10, 150: encoder    -   14, 22, 52, 56: optical circulator    -   20, 184: decoder    -   30, 40, 70, 72: SSFBG    -   32: cladding    -   34: core    -   36: optical fiber    -   38: phase control means    -   41, 42: unit FBG    -   50: optical pulse generator    -   54: evaluation target encoder    -   58: evaluation target decoder    -   60, 62: optical oscilloscope    -   61, 144, 182: splitter    -   140: transmitting section    -   142: pulse light source    -   146: modulated electrical signal production section    -   148: modulator    -   160: encoding section of first channel    -   162: encoding section of second channel    -   164: encoding section of third channel    -   166: encoding section of fourth channel    -   170: multiplexer    -   172: optical transmission line    -   180: receiving section    -   190: photoreceptor    -   200: receiving-section first channel    -   202: receiving-section second channel    -   204: receiving-section third channel    -   206: receiving-section fourth channel

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present invention will be described hereinbelow withreference to the drawings. Each of the drawings shows a constitutionalexample of the present invention. The cross-sectional shape anddispositional relationship and so forth of the respective constituentelements are only shown schematically and the present invention is notlimited to the illustrated examples. Further, although specifiedmaterials and conditions and so forth are sometimes used in thefollowing invention, these materials and conditions represent only oneof the suitable examples and, therefore, are not limited in any way.Further, the same numbers are shown assigned to the same constituentelements in each of the drawings and repeated descriptions are sometimesomitted.

Further, although a case where the phase control means in the first tothird embodiments optical is formed by using optical fiber has beenadopted, the phase control means is not limited to optical fiber and canalso be formed by using a planar-type optical waveguide or the like. Thedecision on whether to use optical fiber as the phase control means orto use a planar-type optical waveguide or the like is merely a designitem. However, because, when an optical pulse time spreader is used asthe optical communication system, the optical communication systememploys optical fiber as the optical transmission line, usage of anoptical pulse time spreader that is constituted by using optical fiberas the phase control means is often suitable.

I. Description of Optical Pulse Time Spreader

First Embodiment

The structure of the phase control means of the optical pulse timespreader constituting the first embodiment of the first invention willnow be described with reference to FIGS. 6(A) and 6(B). FIG. 6(A) is aschematic cross-sectional view of the phase control means. The phasecontrol means have an SSFBG 40 fixed to a core 34 of an optical fiber 36that comprises the core 34 and cladding 32. Fifteen unit FBGs aredisposed in series in the waveguide direction of the core 34, which isthe optical waveguide of the optical fiber 36, to constitute the SSFBG40.

When the optical phase code set for the SSFBG 40 as the phase controlmeans shown in FIG. 6(A) is written as 15 bit code string, the codebecomes (0,0,0,1,1,1,1,0,1,0,1,1,0,0,1). Further, the relationship ofcorrespondence between the fifteen unit FBGs disposed in series in core34 and the optical phase code above is as follows. That is, therespective unit FBGs arranged in a direction from the left end to theright end of the SSFBG 40 shown in FIG. 6(A) and the respective chipsarranged in a direction from the left end to the right end in bracketsrepresenting the optical phase code correspond one for one.

FIG. 6(B) schematically shows the refractive index modulation structureof the SSFBG 40 shown in FIG. 6(A). The horizontal axis is the positioncoordinate in the longitudinal direction of the optical fiber 36 formingthe SSFBG 40. The vertical axis represents the refractive indexmodulation structure of the optical fiber 36 and the difference betweenthe maximum and minimum of the refractive index of the core of theoptical fiber 36 is represented as Δn, where Δn=6.2×10⁻⁵. Further, FIG.6(B) is drawn with the refractive index modulation structure of the core34 of the optical fiber 36 partially enlarged.

The refractive index modulation cycle Λ is 535.2 nm. Further, thewavelength λ of the encoded or decoded optical pulse is 1550 nm and theeffective refractive index of the optical fiber 36 is 1.448. Therefore,the Bragg reflection wavelength is equal to the wavelength λ of theoptical pulse and is set as 1550 nm. That is, because λ=1550 nm,N_(eff)=1.448, and κ=535.2 nm, λ=2N_(eff) Λ=2×1.448×535.2 nm=1549.94nm≈81550 nm is satisfied. Further, the length of the unit FBG is set as2.4 mm.

The relationship of the phases of the Bragg reflected light of theadjacent unit FBGs is established as follows. That is, the phasedifference of the Bragg reflected light from two unit diffractiongratings that adjoin one another from front to back and provide equalcode values is given by2πM+(π/2)  (1)Where M is an integer. Further, the phase difference of the Braggreflected light from the two unit diffraction gratings that adjoin oneanother from front to back and provide different code values are givenby2πM+(2N+1)π+(π/2)  (2)where M and N are integers.

Table 2 shows the relationship between the optical phase code(0,0,0,1,1,1,10,1,0,1,1,0,0,1) of the phase difference the Braggreflected light of adjacent unit FBGs for implementing the optical phasecode. The code values of the optical phase code set for the SSFBG 40constituting the phase control means shown in FIG. 6(A) are shown linedup in a row as the code in the upper level of Table 2. Further, thephase difference of the Bragg reflected light of adjacent unit FBGs isshown as the phase shift amount in the lower level of Table 2. The unitFBGs arranged from the left end to the right end of the SSFBG 40 shownin F. 6A and the chips arranged from the left end to the right end inbrackets representing the optical phase code correspond one for one.TABLE 2

The geometrical interval between adjacent unit FBGs for which the phaseshift amount is π/2 is π/4 when converted to a phase value because thelight travels there and back between the adjacent unit FBGs. Generally,when the geometrical interval between adjacent unit FBGs for which thephase shift amount is π/2 is converted to a phase value, thisgeometrical interval is given by πL+(π/4), where L is an integer. Thatis, the phase difference of the Bragg reflected light from adjacent unitFBGs for which the phase shift is π/2 is given by 2 πL+(π/2). Further,when the geometrical interval between adjacent unit FBGs for which thephase shift amount is 3π/2 is converted to a phase value, thisgeometrical interval is given by πK+(3π/4), where K is an integer andthe phase difference of the Bragg reflected light from the adjacent unitFBGs is given by 2πK+(3π/2).

Further, Table 2 shows −π/2 in brackets when K=−1 and 3π/2 when K=0 incases where the phase difference of the Bragg reflected light is givenby 2πK+(3π/2). These values have substantially the same meaning as phasevalues. Further, by substituting with L=M and K=M+N, it is clear thatthe phase difference satisfies the relationship of Equations (1) and (2)above.

In FIG. 6(A), the interval between adjacent unit FBGs is shown shadedblack. On the other hand, in FIG. 6(B), black triangles are shown addedto the intervals of the adjacent unit FBGs.

The characteristic of the SSFBG 40 constituting the phase control meansof the first embodiment will now be described with reference to FIGS. 7and 8(A) to 8(C). FIG. 7 is a schematic constitutional view of acharacteristic evaluation device that is used to evaluate the functionsof the SSFBG 40. FIGS. 8(A) to 8(C) show the experiment results in acase where the encoding and decoding are executed by using the SSFBG 40of the first embodiment. FIG. 8(A) shows an encoded waveformrepresenting an encoded optical pulse train, FIG. 8(B) shows anautocorrelation waveform, and FIG. 8(C) shows a cross correlationwaveform. In FIGS. 8(A) to 8(C), the horizontal axis is shown calibratedwith time in ps units and the vertical axis is shown calibrated with theoptical intensity on an optional scale.

First, the constitution of the characteristic evaluation device that isused to evaluate the functions of the SSFBG 40 will be described withreference to FIG. 7. The characteristic evaluation device is constitutedcomprising an optical pulse generator 50, optical circulators 52 and 56,and optical oscilloscopes 60 and 62. Further, an evaluation targetencoder 54 is connected to the optical circulator 52 and an evaluationtarget decoder 58 is connected to the optical circulator 56. Further, asplitter 61 is provided midway along the optical fiber linking theoptical circulator 52 and optical circulator 56 in order to observe anencoded waveform 53S. The splitter 61 is constituted to split a portionof the encoded waveform 53S and supply same to the optical oscilloscope60.

An encoded waveform representing the encoded optical pulse train isobserved by the optical oscilloscope 60 provided in the characteristicevaluation device shown in FIG. 7 and an autocorrelation waveform and across correlation waveform are observed by the optical oscilloscope 62.

An optical pulse 51S is produced by the optical pulse generator 50before being propagated by a transmission line 51 and input to theevaluation encoder 54 via the optical circulator 52. The optical pulse51S is encoded by the evaluation encoder 54 to produce an encodedoptical pulse train 53S that is propagated once again by thetransmission line 53 via the optical circulator 52. The encoded opticalpulse train 53S is split by the splitter 61 and supplied to the opticaloscilloscope 60 and observed. The encoded optical pulse train 53S isinput to the evaluation decoder 58 via the optical circulator 56. Theencoded optical pulse train 53S is decoded by the evaluation decoder 58to produce an autocorrelation waveform (or cross correlation waveform)57S which is propagated once again by a transmission line 57 via theoptical circulator 56 before being supplied to the optical oscilloscope62 and observed. Schematic shapes are shown surrounded by squares inFIG. 7 for encoded waveforms that represent the optical pulse 51S andencoded optical pulse train 53S and a decoded waveform that representsthe autocorrelation waveform (or cross correlation waveform) 57S.

The half width of the optical pulse used in the characteristicevaluation of the SSFBG 40 of the first embodiment is 20 ps. That is,the half width of the optical pulse 51S produced by the optical pulsegenerator 50 is 20 ps. Further, the input end and output end of theSSFBG 40 are provided mutually reversed with respect to the encoder anddecoder in order to obtain the autocorrelation waveform. In addition,the input end and output end of the SSFBG 40 are provided the same asone another with respect to the encoder and decoder in order to obtainthe cross correlation waveform.

That is, two SSFBGs 40, which are phase control means with the samerefractive index modulation structure, that is, SSFBGs 40 for which thesame codes have been set, are fabricated, one of which is the evaluationencoder 54 and the other of which is the evaluation decoder 58.Therefore, the autocorrelation waveform was observed by installing unitFBGs so that the order of arrangement of unit FBGs that are arrangedfrom the one end that faces the optical circulator 52 of the evaluationencoder 54 toward the other end and the order of arrangement of the unitFBGs that are arranged from the one end facing the optical circulator 56of the evaluation decoder 58 toward the other end are reversed. Further,the cross correlation waveform was observed by installing unit FBGs sothat the order of arrangement of unit FBGs that are arranged from theone end that faces the optical circulator 52 of the evaluation encoder54 toward the other end and the order of arrangement of the unit FBGsthat are arranged from the one end facing the optical circulator 56 ofthe evaluation decoder 58 toward the other end are the same.

The area ratio with respect to the encoded waveform representing theencoded optical pulse train 53S shown in FIG. 8(A) is 0.16. This issubstantially the same size as the area ratio 0.15 with respect to theencoded waveform obtained by a conventional SSFBG. However, incomparison with the encoded waveform obtained by the conventional SSFBGshown in FIG. 4(A), variations in the intensity of the encoded opticalpulse train are small. For this reason, as mentioned hereinbelow, theP/W value and P/C value are large.

P/W=19.0 for the decoded waveform representing the autocorrelationwaveform shown in FIG. 8(B) and P/C=4.5 for the decoded waveformrepresenting the cross correlation waveform shown in FIG. 8(C). On theother hand, the P/W value and P/C value in a case where encoding anddecoding are performed by means of a conventional SSFBG are P/W=11.8 andP/C=2.7 respectively. It can be seen from this that a large value isobtained for both P/W and P/C when encoding and decoding are performedby using the optical pulse time spreader of the first embodiment.

It can be seen that these values approach the maximum values for thevalues of P/W and P/C described with reference to FIGS. 5(A) and 5(B).The values of both P/W and P/C described with reference to FIGS. 5(A)and 5(B) are values that are calculated when encoding/decoding has takenplace in an ideal state in which time intervals between chip pulsesconstituting the encoded optical pulse train are sufficiently spacedapart and variations in the intensity of the encoded optical pulse trainarising from interference between the chip pulses can be ignored.

The fact that the P/W value is large signifies that the peak of theautocorrelation waveform can be easily identified. Further, the factthat the P/C value is large signifies that separation of theautocorrelation waveform and the cross correlation waveform isstraightforward. Therefore, with the optical code division multiplexingtransmission device that uses the optical pulse time spreader of thefirst embodiment, the cross correlation waveform component from thedecoded optical pulse signal is separated from the autocorrelationwaveform component and the identification conditions set for thejudgment circuit for identifying the autocorrelation waveform arerelaxed.

Further, in the first embodiment, the relationship of the phases of theBragg reflected light of adjacent unit FBGs was described by adoptingthe cases given by Equations (1) and (2) above. However, the opticalpulse time spreaders contained in the technological scope of the presentinvention is not limited to a case where the relationship of the phasesof the Bragg reflected light of the adjacent unit FBGs is strictly givenmathematically by Equations (1) and (2) above. That is, this means thatthe best results are exhibited by the optical pulse time spreader of thepresent invention if the relationship of the phases of the Braggreflected light of adjacent unit FBGs is strictly formed so as to beequal to the values given by Equations (1) and (2) above.

Even if fabrication involves a design such that the relationship of thephase of the Bragg reflected light of adjacent unit FBGs is given byEquations (1) and (2) above, fabrication errors in the fabrication stepsand variations and so forth in the effective refractive index of theoptical fiber constituting the material for fabricating the SSFBGconstituting the phase control means exist. Therefore, the SSFBG isdesigned with the relationship given by Equations (1) and (2) aboveserving as design pointers. The fabricated SSFBG has a width of anaccuracy based on the fabrication errors and so forth in the fabricationprocess and as long as the phase relationship given by Equations (1) and(2) above is satisfied, the SSFBG is naturally included in thetechnological scope of the phase control means of the present invention.

Furthermore, in the above embodiment, a constitution in which the phasedifference of the output light of adjacent equal code values satisfiesEquation (1) above and the phase difference of the output light ofadjacent different code values satisfies Equation (2) above wasdescribed but, as is clear from Equations (1) and (2) above, becausediscrimination of whether adjacent code values are equal or different isfavorable if the difference in phase difference of the respective outputlight satisfies (2N+1)π, the same effect can be obtained even with aconstitution in which the phase difference of the output light ofadjacent different code values satisfies Equation (1) above and thephase difference of output light of adjacent equal code values satisfiesEquation (2) above.

In addition, in the above embodiment, although a constitution in whichunit diffraction gratings, which correspond one for one with code valuesconstituting the optical code that are arranged in a row, are arrangedin series in the waveguide direction of the optical waveguide is usedfor the phase control means for generating chip pulses, the presentinvention is not limited to or by such a constitution.

For example, a transversal-type filter structure or the like that isformed by planar waveguide circuit technology is used as the phasecontrol means and the same results can be obtained by creating a designsuch that the phase difference of the output light of adjacent equalcode values satisfies Equation (1) above and the phase difference of theoutput light of adjacent different code values satisfies Equation (2)above or a design such that the phase difference of the output light ofadjacent different code values satisfies Equation (1) above and thephase difference of the output light of adjacent equal code valuessatisfies Equation (2) above.

Second Embodiment

The structure of the SSFBG of the phase control means of the opticalpulse time spreader constituting the second embodiment of the firstinvention will now be described with reference to FIGS. 9(A) and 9(B).FIG. 9(A) is a schematic cross-sectional view of the phase controlmeans. The phase control means has a structure in which an SSFBG 70 isfixed to the core 34 of the optical fiber 36 comprising the core 34 andcladding 32. Fifteen unit FBGs are arranged in series in the waveguidedirection of the core 34 constituting the optical waveguide of theoptical fiber 36 to constitute the SSFBG 70. Because only the refractiveindex modulation structure of the SSFBG 70 is different and the otherparts are the same as those of the optical pulse time spreader of thefirst embodiment, repeated descriptions are omitted here. The opticalphase code set for the phase control means of the optical pulse timespreader of the second embodiment is the same as the optical phase codeset for the phase control means of the first embodiment.

The fact that the refractive index modulation structure of the SSFBG 70differs from the refractive index modulation structure of the SSFBG 40of the first embodiment means that the intensity of the refractive indexmodulation of the periodic refractive index modulation structure of theunit FBG constituting the SSFBG 70 is apodized by means of a windowfunction. In the second embodiment, a Gaussian error function is adoptedas the window function.

FIG. 9(B) schematically shows the refractive index modulation structureof the SSFBG 70 shown in FIG. 9(A). Further, FIG. 9(B) also hasenlargements of a portion of the refractive index modulation structureof the unit FBG. A description of a method of apodizing the refractiveindex modulation intensity of the periodic refractive index modulationstructure of the unit FBG by means of a window function will now bedescribed with reference to the part of FIG. 9(B) in which a portion ofthe refractive index modulation structure of the unit FBG is enlarged.

The periodic refractive index modulation structure of the unit FBG priorto apodization is fixed in the optical waveguide direction (x direction)of the optical fiber 36 the amplitude of which is given by Δn/2, asshown on the far right of FIG. 9(B). That is, the periodic refractiveindex modulation structure of the unit FBG prior to apodization is givenby the following Equation (3):(Δn/2)·sin(2πx/Λ)  (3)

Here, x is the position coordinate in the longitudinal direction of theoptical fiber 36.

A unit FBG with a periodic refractive index modulation structure that isgiven by a new function rendered by multiplying Equation (3) by a windowfunction which is given by the following Equation (4) is known as a unitFBG that is apodized by a function given by Equation (4).exp[−Ln2[2(x−(L/2))/LB]²]  (4)

Here, Ln2 signifies the natural logarithm of 2. Further, exp signifiesan exponential function in which the base of the natural logarithm is anindex. Further, in the second embodiment, the settings are Δn=1.23×10⁻⁴,L=2.346 mm, B=0.5.

By performing such apodization, Bragg reflection arises in concentrationin the middle of the unit FBG and, as a result, a narrowing of the halfwidth of the time waveform of the Bragg reflected light thus generatedis expected. That is, because a narrowing of the half width of the chippulses constituting the encoded optical pulse train is expected, it isexpected that it will be possible to reduce the overlap of the feet ofthe chip pulses constituting the encoded optical pulse train on the timeaxis. If the overlap of the feet of the chip pulses can be reduced, theeffect of interference caused by the overlap of the feet of the chippulses on the time axis can be reduced as mentioned earlier. As aresult, in comparison with a case where encoding and decoding areperformed by using an optical pulse time spreader of the firstembodiment, larger values can be expected for both P/W and P/C.

Further, the function for the apodization corresponding with Equation(4) is not limited to a Gaussian function. The size of the amplitude ofthe periodic refractive index modulation structure of the unit FBG givenby Equation (3) can be adopted as long as the size is a functionpermitting apodization with a maximum in the middle part of the unitFBG. For example, it is also possible to use functions that are used insignal processing technological fields such as the Raised cosine, Tanh,Blackman, Hamming, and Hanning functions and so forth, for example.

FIGS. 10(A) to 10(C) show the experiment results in a case whereencoding and decoding are executed by using the optical pulse timespreader of the second embodiment. FIG. 10(A) shows an encoded waveformrepresenting an encoded optical pulse train, FIG. 10(B) shows anautocorrelation waveform, and FIG. 10(C) shows a cross correlationwaveform. In FIGS. 10(A) to 10(C), the horizontal axis shows timecalibrated in ps units and the vertical axis shows the optical intensitycalibrated with an optional scale.

The device whose constitution was described with reference to FIG. 7 wasused in the characteristic evaluation of the optical pulse time spreaderof the second embodiment in the same way as when the characteristicevaluation of the optical pulse time spreader of the first embodimentwas performed. Further, the half width of the optical pulse used in thecharacteristic evaluation of the optical pulse time spreader of thesecond embodiment was 40 ps.

The area ratio with respect to the encoded waveform representing theencoded optical pulse train shown in FIG. 10(A) is 0.19. This is a valuethat is on the order of 27% larger than the area ratio 0.15 with respectto the encoded waveform obtained by conventional phase control means.This means that the energy of the optical pulse is effectively convertedto the encoded optical pulse train. That is, this means that encoding isefficiently performed and shows that the optical pulse time spreader ofthe second embodiment is more suitably used as an encoder.

As shown in FIG. 10(B), P=6.53, W=0.384. Further, as shown in FIG.10(C), C=1.81. As a result, for the decoded waveform representing theautocorrelation waveform shown in FIG. 10(B), P/W=17.0 and, for thedecoded waveform representing the cross correlation waveform shown inFIG. 10(C), P/C=3.6. On the other hand, the P/W value and P/C value whenencoding and decoding are performed by conventional phase control meansare P/W=11.8 and P/C=2.7 respectively. It is clear from this that largevalues are obtained for both P/W and P/C when encoding and decoding areperformed by using the optical pulse time spreader of the secondembodiment.

It can be seen that these values approach the maximum values for bothP/W and P/C values described with reference to FIGS. 5(A) and 5(B).

Further, for the purpose of a comparison, an example in which encodingand decoding are performed by using conventional phase control means asthe encoder and decoder is shown in FIGS. 11(A) to 11(C). Here, the halfwidth of the optical pulse used in the characteristic evaluation was 40ps. That is, the example described with reference to FIGS. 4(A) to 4(C)is an example in which encoding and decoding were performed by using anoptical pulse time spreader comprising conventional phase control meansas the encoder and decoder. The half width of the optical pulse used inthe characteristic evaluation was set at 20 ps, which is half the halfwidth 40 ps.

The area ratio with respect to the encoded waveform representing theencoded optical pulse train shown in FIG. 11(A) is 0.15. This equals thearea ratio with respect to the encoded waveform obtained by the opticalpulse time spreader that comprises conventional phase control means.This means that the energy conversion efficiency from the optical pulseto the encoded optical pulse train does not change.

As shown in FIG. 11(B), P=4.72 and W=0.628. Further, as shown in FIG.11(C), C=3.12. As a result, in the decoded waveform representing theautocorrelation waveform shown in FIG. 11(B), P/W=7.5 and in the decodedwaveform representing the cross correlation waveform shown in FIG.11(C), P/C=1.5. On the other hand, the P/W value and P/C value in a casewhere encoding and decoding are performed by the optical pulse timespreader of the second embodiment are P/W=17.0 and P/C=3.6. It can beseen from this that, when encoding and decoding are performed by usingthe optical pulse time spreader of the second embodiment, the half valueof the optical pulse used in the characteristic evaluation equals 40 psand large values are obtained for both P/W and P/C in comparison withthe characteristic of the conventional phase control means.

Third Embodiment

The structure of the phase control means of the optical pulse timespreader constituting the third embodiment of the first invention willnow be described with reference to FIGS. 12(A) and 12(B). FIG. 12(A) isa schematic cross-sectional view of the optical pulse time spreader. Theoptical pulse time spreader is a structure in which the SSFBG 72 isfixed to the core 34 of the optical fiber 36 that comprises the core 34and cladding 32. Fifteen unit FBGs are disposed in series in thewaveguide direction of the core 34, which is the optical waveguide ofthe optical fiber 36, to constitute the SSFBG 72. Because only therefractive index modulation structure of the SSFBG 72 is different andthe other parts are the same as those of the optical pulse time spreaderof the first embodiment, repeated descriptions are omitted here. Theoptical phase code set for the phase control means of the optical pulsetime spreader of the third embodiment is the same as the optical phasecode set for the phase control means of the first embodiment.

The fact that the refractive index modulation structure of the SSFBG 72differs from the refractive index modulation structure of the SSFBG 40of the first embodiment means that the intensity of the refractive indexmodulation of the periodic refractive index modulation structure of theunit FBG constituting the SSFBG 72 is set as follows.

That is, this embodiment is characterized by a constitution whereby therefractive index modulation intensity of the periodic refractive indexmodulation structure in which unit FBGs arranged in series in thewaveguide direction of the optical fiber are formed increasesmonotonously in the waveguide direction of the optical waveguide. Morespecifically, in the case of the SSFBG constituting the phase controlmeans that comprises fifteen unit FBGs, numbers from a first number to afifteenth number are assigned in order from one end to the other of theoptical fiber to the unit FBGs and the reflectance R_(i) from the ith(2≦i≦15) unit FBG is given byR _(i) =R _(i-1)/(1−R _(i-1))²  (5)

FIG. 12(B) schematically shows the refractive index modulation structureof the SSFBG 72 shown in FIG. 12(A). Further, FIG. 12(B) showsenlargements of a portion of the refractive index modulation structureof the unit FBG. The enlargement of a portion of the refractive indexmodulation structure of the unit FBG equals the refractive indexmodulation structure of the phase control means of the first embodimentshown in FIG. 6(B). However, the amplitude increases monotonously in theoptical waveguide direction (x direction) of the optical fiber 36. InFIG. 12(B), numbers from 1 to 15 are assigned in order to identify theunit FBGs.

The modulation intensity amplitude Δn of the refractive index modulationstructure of the first unit FBG (i=1) is 8.2×10⁻⁵ and the Braggreflectance R₁ is 0.0238. Further, the modulation intensity amplitude Δnof the refractive index modulation structure of the fifteenth unit FBG(i=15) is 1.46×10⁻⁴ and the Bragg reflection efficiency R₁ is 0.0688.Table 3 shows the modulation intensity amplitude Δn and Braggreflectance R_(i) (i=1, 2, . . . , 15) of the refractive indexmodulation structure of the first to fifteenth unit FBGs in the form ofa list. The Bragg reflectance R_(i) satisfies Equation (5) above. As perTable 3, the value of Δn and R_(i) are such that the amplitude increasesmonotonously in the optical waveguide direction (x direction) of theoptical fiber 36. TABLE 3 Unit FBG Δn Reflectance 1  8.2 × 10⁻⁵ 0.0238 2 8.4 × 10⁻⁵ 0.0250 3  8.7 × 10⁻⁵ 0.0263 4  9.0 × 10⁻⁵ 0.0277 5  9.3 ×10⁻⁵ 0.0293 6  9.6 × 10⁻⁵ 0.0311 7  9.9 × 10⁻⁵ 0.0331 8 1.03 × 10⁻⁴0.0354 9 1.08 × 10⁻⁴ 0.0381 10 1.12 × 10⁻⁴ 0.0412 11 1.17 × 10⁻⁴ 0.044812 1.23 × 10⁻⁴ 0.0491 13 1.30 × 10⁻⁴ 0.0543 14 1.37 × 10⁻⁴ 0.0607 151.46 × 10⁻⁴ 0.0688

Strictly speaking, the individual unit FBGs may be constituted such thatthe modulation intensity amplitude Δn of the refractive index modulationstructure is increased monotonously in the waveguide direction of theoptical fiber but the effects mentioned hereinbelow are adequatelyobtained as long as the constitution is such that the Bragg reflectanceR_(i) satisfies Equation (5).

If the design is such that the Δn and Bragg reflectance R_(i) of therefractive index modulation structure of the first to fifteenth unitFBGs are afforded the values shown in Table 3, the Bragg reflected lightintensities from the first to fifteenth unit FBGs can all be equalized,as mentioned hereinbelow.

The optical pulse that is input to the encoder and the encoded opticalpulse train that is input to the decoder are both Bragg-reflected by thefirst unit FBG and, at the stage where same are to enter the second unitFBG, the intensity decreases by the intensity of the Bragg reflectedlight of the first unit FBG. As a result, when the reflectance of thefifteen unit FBGs are all set equal, the intensity of the Braggreflected light of the second unit FBG is smaller than the intensity ofthe Bragg reflected light of the first unit FBG. Thus, the intensity ofthe Bragg reflected light from the respective unit FBGs weakenssequentially in the order of the first to fifteenth unit FBGs.

Therefore, by establishing a constitution in which the reflectancemodulation intensity of the fifteen unit FBGs disposed in series in thewaveguide direction of the optical fiber is increased monotonously inthe waveguide direction, the Bragg reflectance of the respective unitFBGs is set to monotonously increase sequentially in order from thefirst to the fifteenth unit FBGs. Thus, the Bragg reflectance can beincreased to compensate for the decrease in the incident intensity tothe respective unit FBGs and the Bragg reflected light intensities fromthe first to fifteenth unit FBGs can all be equalized.

If the Bragg reflected light intensities from the first to fifteenthunit FBGs can all be equalized, the time waveform of the encoded opticalpulse train can be approximated to a shape that is smooth with respectto the time axis. In other words, this means that the encoded opticalpulse is time-spread uniformly within the spreading time by the encoder.When the optical pulse is time-spread uniformly within the spreadingtime, the energy of the optical pulse is more efficiently converted toan encoded optical pulse train in comparison with a case wherenonuniform time spreading is performed. Further, as the subsequentexperiment results show, larger values are obtained for both P/W andP/C.

FIGS. 13(A) to 13(C) shows the experiment results in a case whereencoding and decoding are performed by using the optical pulse timespreader of the third embodiment. FIG. 13(A) shows an encoded waveformrepresenting an encoded optical pulse train, FIG. 13(B) shows anautocorrelation waveform, and FIG. 13(C) shows a cross correlationwaveform. In FIGS. 13(A) to 13(C), the horizontal axis shows timecalibrated in ps units and the vertical axis shows the optical intensitycalibrated with an optional scale.

The device whose constitution was described with reference to FIG. 7 wasused in the characteristic evaluation of the optical pulse time spreaderof the third embodiment in the same way as when the characteristicevaluation of the optical pulse time spreader of the first embodimentwas performed. Further, the half width of the optical pulse used in thecharacteristic evaluation of the optical pulse time spreader of thethird embodiment was 20 ps.

The area ratio with respect to the encoded waveform representing theencoded optical pulse train shown in FIG. 13(A) is 0.39. This is a valuethat is 2.6 times larger than the area ratio 0.15 with respect to theencoded waveform obtained by an optical pulse time spreader comprisingconventional phase control means. This means that the energy of theoptical pulse is effectively converted to the encoded optical pulsetrain. That is, this means that encoding is efficiently performed andshows that the optical pulse time spreader of the third embodiment ismore suitably used as an encoder.

As shown in FIG. 13(B), P=4.36, W=0.217. Further, as shown in FIG.13(C), C=0.999.

For the decoded waveform representing the autocorrelation waveform shownin FIG. 13(B), P/W=20.1 and, for the decoded waveform representing thecross correlation waveform shown in FIG. 13(C), P/C=4.4. On the otherhand, the P/W value and P/C value when encoding and decoding areperformed by an optical pulse time spreader comprising conventionalphase control means are P/W=11.8 and P/C=2.7 respectively. It is clearfrom this that large values are obtained for both P/W and P/C whenencoding and decoding are performed by using the optical pulse timespreader of the third embodiment.

It can be seen that these values approach the maximum values for bothP/W and P/C values described with reference to FIGS. 5(A) and 5(B).

Further, for the purpose of a comparison, an example in which encodingand decoding are performed by using an optical pulse time spreadercomprising conventional phase control means as the encoder and decoderis shown in FIGS. 14(A) to 14(C). Here, the experiment was performed byusing a conventional optical pulse time spreader that is designed sothat the reflectance is two times the reflectance of the SSFBGconstituting the conventional phase control means shown in FIG. 2. Thisserves to compare the characteristic with that of the SSFBG of aconventional optical pulse time spreader under the condition of beingsubstantially equal to the Bragg reflectance of the SSFBG constitutingthe phase control means of the optical pulse time spreader of the thirdembodiment.

The area ratio with respect to the encoded waveform representing theencoded optical pulse train shown in FIG. 14(A) is 0.45. This is largerthan the area ratio 0.39 with respect to the encoded waveform obtainedby the optical pulse time spreader of the third embodiment. Therefore,the time waveform of the encoded optical pulse train has a shape ofsharp asperity with respect to the time axis. That is, this means thatthe encoded optical pulse is time-spread nonuniformly within thespreading time by the encoder. As a result, the value of P/W and P/C issmaller than the value with respect to the optical pulse time spreaderof the third embodiment as shown hereinbelow.

As shown in FIG. 14(B), P=5.70 and W=0.716. Further, as shown in FIG.14(C), C=2.33. As a result, in the decoded waveform representing theautocorrelation waveform shown in FIG. 14(B), P/W=8.0 and, in thedecoded waveform representing the cross correlation waveform shown inFIG. 14(C), P/C=2.4. On the other hand, the P/W value and P/C value in acase where encoding and decoding are performed by the optical pulse timespreader of the third embodiment are P/W=20.1 and P/C=4.4 respectively.It can be seen from this that, when encoding and decoding are performedby using the optical pulse time spreader of the third embodiment, largevalues are also obtained for both P/W and P/C in comparison with thecharacteristic of a conventional optical pulse time spreader byequalizing the Bragg reflectance.

II. Description of the Optical Code Division Multiplexing TransmissionMethod and Device

The optical pulse time spreaders of the first to third embodiments aresuitable when applied to an optical code division multiplexingtransmission method (called the ‘OCDM transmission method’ hereinafter).That is, by adopting the optical pulse time spreader of the presentinvention as an encoder and decoder, an OCDM transmission methodcomprising the following steps can be implemented. An OCDM transmissionmethod that is implemented by using the optical pulse time spreader ofthe present invention reflects the characteristic that large values areobtained for both the P/W and P/C above. Therefore, even when theintensity of the encoded optical pulse train decreases while same ispropagated by the optical transmission line or optical noise invades theoptical transmission line, the autocorrelation waveform peak can beextracted highly reliably through decoding. That is, an OCDMtransmission method for which high reliability is secured isimplemented.

The OCDM transmission method for which the optical pulse time spreaderof the present invention is suitably used as the encoder and decodercomprises an encoding step of encoding an optical pulse signal by usingoptical phase code to generate the optical pulse signal as an encodedoptical pulse signal; and a decoding step of decoding the encodedoptical pulse signal by using the same code as the optical phase code togenerate an autocorrelation waveform of the optical pulse signal.Further, the encoding step and the decoding step are executed by usingthe optical pulse time spreader of the present invention.

The OCDM transmission method can be implemented by the optical codedivision multiplexing transmission device (‘OCDM transmission device’subsequently) that will be described subsequently. That is, the OCDMtransmission device comprises an encoder that encodes an optical pulsesignal by using optical phase code to generate an encoded optical pulsesignal and a decoder that decodes the encoded optical pulse signal byusing the same code as the optical phase code to generate anautocorrelation waveform of the optical pulse signal. That is, theencoding step is implemented by the encoder and the decoding step isimplemented by the decoder. The optical pulse time spreader of thepresent invention is used as the encoder and decoder.

A suitable constitution for the OCDM transmission device constitutingthe second invention and the functions thereof will be described byusing the optical pulse time spreader of the first invention describedwith reference to FIGS. 6 to 14 (optical pulse time spreader of thefirst to third embodiments) will now be described with reference to FIG.15. In FIG. 15, optical signal paths such optical fibers are indicatedby bold lines and electrical signal paths are indicated by narrow lines.Further, the numbers assigned to the bold lines and narrow lines signifythe optical signals or electrical signals propagated by the respectivepaths in addition to indicating the paths.

FIG. 15 shows an OCDM transmission device of a four-channel constitutionby way of example but the OCDM transmission device is not limited tofour channels. The following description is similarly valid irrespectiveof the number of channels in the constitution.

The OCDM transmission device is constituted to generate an encodedoptical pulse signal for each channel in a transmitting section 140 andmultiplexes the encoded optical pulse signals of all the channels bymeans of a multiplexer 170 to produce a transmission signal 172 s andtransmits the transmission signal 172 s to a receiving section 180 viaan optical transmission line 172.

The transmission signal 172 s, which is produced by multiplexing theencoded optical pulse signals of all the channels transmitted to thereceiving section 180, is intensity-divided in a number equal to thenumber of channels as encoded optical pulse signals by a splitter 182.Further, each of the intensity-divided encoded optical pulse signals 181a, 181 b, 181 c, and 181 d are input to a receiving-section firstchannel 200, a receiving-section second channel 202, a receiving-sectionthird channel 204, and a receiving-section fourth channel 206 of thereceiving section 180.

First, the functional part that produces base optical pulse trains forgenerating optical pulse signals which are the transmission signals ofeach channel and supplies the optical pulse trains to the respectivechannels will be described. This part is constituted comprising a pulselight source 142 and a splitter 144.

The pulse light source 142 can be constituted by using a distributedfeedback semiconductor laser, for example. The light source, which isconstituted to convert continuous wave light output by the DFB-LD intoan optical pulse train by means of an optical modulator (not shown) andto output the optical pulse train from one optical fiber end, is a pulselight source 142. The output light 143 of the pulse light source 142 isintensity-divided for a number of channels (four here) by the splitter144 and distributed to the respective channels. That is, the outputlight 143 is intensity-divided as the optical pulse train 145 a, opticalpulse train 145 b, optical pulse train 145 c, and optical pulse train145 d and supplied to the first to fourth channels respectively.

Because the subsequent description of the encoding section is common toeach channel, the description will be provided here by taking the firstchannel as an example. A transmitting-section first channel 160, whichis the encoding section of the first channel, is constituted comprisinga modulated electrical signal production section 146, a modulator 148,and an encoder 150. The second channel 162, third channel 164, andfourth channel 166 have the same structure as the first channel 160. Thedifference lies with optical phase code that is set for the encoder thateach channel comprises. The optical phase code is set differently foreach channel. As a result, optical pulse signals can be sent andreceived independently for each channel. With the exception of theencoder, the first to fourth channels all have the same structure.

The encoding section is a part that executes an encoding step ofgenerating an encoded optical pulse signal by using optical phase codeto encode an optical pulse signal that contains light of differentwavelengths in a number equal to the number of channels.

As mentioned earlier, the required constituent elements for constitutingthe encoding section 160 are the modulated electrical signal productionsection 146, modulator 148, and encoder 150. The modulated electricalsignal production section 146 executes a step of producing an electricalpulse signal 147 that represents a transmission signal. The electricalpulse signal 147 is an electrical signal that is generated as a binarydigital electrical signal in which transmission information allocated tothe first channel is reflected.

The modulator 148 executes a step of converting the optical pulse train145 a to an optical pulse signal 149 by means of the electrical pulsesignal 147. The optical pulse train 145 a is intensity-modulated to theRZ format that reflects the electrical pulse signal 147 by the modulator148 and generated as the optical pulse signal 149.

The encoder 150 executes a step of encoding the optical pulse signal 149by using optical phase code to generate an encoded optical pulse signal161. The encoder 150 is provided with an optical pulse time spreader ofthe present invention, which has a function to encode the optical pulsesignal 149 by means of the optical phase code to generate an encodedoptical pulse signal 161. Further, the decoder 184 provided in thereception-channel first channel 200 of the receiving section 180 alsocomprises an optical pulse time spreader for which the same opticalphase code as the optical phase code set for the encoder 150 has beenset.

The decoder 184 decodes the encoded optical pulse signal 181 a allocatedto the first intensity-divided channel by using the same code as theoptical phase code set for the encoder 150 of the first channel. As aresult, the decoder 184 generates a playback optical pulse signalcomprising an autocorrelation waveform component of the optical pulsesignal of the first channel and a cross correlation waveform componentof the optical pulse signal of the second to fourth channels.

That is, the decoder 184 extracts only an autocorrelation waveformcomponent 185 of the optical pulse signal of the first channel. Theautocorrelation waveform component 185 is converted into an electricalsignal by a photoreceptor 190 to generate a reception signal 191 of thefirst channel. The waveform of the reception signal 191 is a signal thatreflects the electrical pulse signal 147 output by the modulatedelectrical signal production section 146 that the encoding section 160of the first channel of the transmitting section 140 comprises. Thus,the electrical pulse signal 147 that is to be transmitted via the firstchannel is received as the reception signal 191 of the first channel bythe receiving section 180.

The OCDM transmission method and OCDM transmission device constitutingthe second invention are implemented by using the optical pulse timespreader of the first invention. Therefore, according to the OCDMtransmission method and OCDM transmission device constituting the secondinvention, the intensity of the encoded optical pulse train decreases asa result of light absorption of the optical fiber constituting thetransmission line and, even though optical noise produced by an opticalamplifier that is integrated into the device if required invades theencoded optical pulse train, the autocorrelation waveform can beextracted highly reliably. That is, if the OCDM transmission device isconstituted by using the optical pulse time spreader constituting thefirst invention, the P/W value can be increased and, therefore,identification of the peak of the autocorrelation waveform isstraightforward. Further, because the P/C value can also be made large,the autocorrelation waveform and cross correlation waveform can beeasily separated.

Therefore, the OCDM transmission device, which uses the optical pulsetime spreader of the first to third embodiments of the presentinvention, affords the effect of being able to separate the crosscorrelation waveform component from the decoded optical pulse signalfrom the autocorrelation waveform and relax the identificationconditions set for the judgment circuit for identifying theautocorrelation waveform.

Moreover, because the characteristic of the encoder is approximated tothe results of an ideal code correlation simulation, the system designis straightforward when the OCDM transmission device is used.

1. An optical pulse time spreader that time-spreads an optical pulse as a series of chip pulse stream that are sequentially arranged on the time axis by means of encoding that employs optical phase code and outputs said series of chip pulse stream, comprising: phase control means that generate said series of chip pulse stream by providing a phase difference between adjacent chip pulses among code values constituting said optical phase code, wherein, when said adjacent code values are equal, said phase control means give the phase difference by 2πM+(π/2)  (1), and, when said adjacent code values are different, said phase control means give the phase difference by 2πM+(2N+1)π+(π/2)  (2) (where M and N are integers).
 2. An optical pulse time spreader that time-spreads an optical pulse as a series of chip pulse stream that are sequentially arranged on the time axis by means of encoding that employs optical phase code and outputs said series of chip pulse stream, comprising: phase control means that generate said series of chip pulse stream by providing a phase difference between adjacent chip pulses among code values constituting said optical phase code, wherein, when said adjacent code values are different, said phase control means give the phase difference by 2πM+(π/2)  (1), and, when said adjacent code values are equal, said phase control means give the phase difference 2πM+(2N+1)π+(π/2)  (2) (where M and N are integers).
 3. An optical pulse time spreader that time-spreads an optical pulse as a series of chip pulse stream that are sequentially arranged on the time axis by means of encoding that employs optical phase code and outputs said series of chip pulse stream, comprising: phase control means for generating said series of chip pulse stream, wherein unit diffraction gratings that are arranged in a row and correspond one for one with code values constituting said optical phase code are arranged in series in the waveguide direction of an optical waveguide; the phase difference of Bragg reflected light from two unit diffraction gratings that adjoin one another from front to back and provide equal code values is given by: 2πM+(π/2)  (1); and the phase difference of Bragg reflected light from two unit diffraction gratings that adjoin one another from front to back and provide different code values is given by: 2πM+(2N+1)π+(π/2)  (2) (where M and N are integers).
 4. An optical pulse time spreader that time-spreads an optical pulse as a series of chip pulse stream that are sequentially arranged on the time axis by means of encoding that employs optical phase code and outputs said series of chip pulse stream, comprising: phase control means for generating said series of chip pulse stream, wherein unit diffraction gratings that are arranged in a row and correspond one for one with code values constituting said optical phase code are arranged in series in the waveguide direction of an optical waveguide; the phase difference of Bragg reflected light from two unit diffraction gratings that adjoin one another from front to back and provide different code values is given by: 2πM+(π/2)  (1); and the phase difference of Bragg reflected light from two unit diffraction gratings that adjoin one another from front to back and provide equal code values is given by: 2πM+(2N+1)π+(π/2)  (2) (where M and N are integers).
 5. The optical pulse time spreader according to claim 3, wherein the refractive index modulation intensity of the periodic refractive index modulation structure of said unit diffraction grating is apodized by means of a window function.
 6. (canceled)
 7. The optical pulse time spreader according to claim 3, wherein the refractive index modulation intensity of the periodic refractive index modulation structure in which said unit diffraction gratings arranged in series in the waveguide direction of said optical waveguide are formed is monotonously increased in said waveguide direction of the optical waveguide.
 8. The optical pulse time spreader according to claim 3, comprising: phase control means comprising a number J (J is a natural number of 2 or more) of unit diffraction gratings, wherein numbers from first to Jth are assigned to said unit diffraction gratings sequentially from one end of said optical waveguide to the other end thereof; and the reflectance Ri from the ith (2≦i≦J) unit diffraction grating is given by reflectance Ri=Ri−1/(1−Ri−1)2 (3).
 9. (canceled)
 10. An optical code division multiplexing transmission method, comprising: an encoding step of encoding an optical pulse signal by using optical phase code to generate said optical pulse signal as an encoded optical pulse signal; and a decoding step of decoding said encoded optical pulse signal by using said same code as said optical phase code to generate an autocorrelation waveform of the optical pulse signal, wherein said encoding step and said decoding step are executed by using the optical pulse time spreader according to claim
 1. 11. An optical code division multiplexing transmission device, comprising: an encoder that encodes an optical pulse signal by using optical phase code to generate said optical pulse signal as an encoded optical pulse signal; and a decoder that decodes said encoded optical pulse signal by using said same code as the optical phase code to generate an autocorrelation waveform of said optical pulse signal, wherein said encoder and said decoder are optical pulse time spreaders according to claim
 1. 12. The optical pulse time spreader according to claim 4, wherein the refractive index modulation intensity of the periodic refractive index modulation structure of said unit diffraction grating is apodized by means of a window function.
 13. The optical pulse time spreader according to claim 4, wherein the refractive index modulation intensity of the periodic refractive index modulation structure in which said unit diffraction gratings arranged in series in the waveguide direction of said optical waveguide are formed is monotonously increased in said waveguide direction of the optical waveguide.
 14. The optical pulse time spreader according to claim 4, comprising: phase control means comprising a number J (J is a natural number of 2 or more) of unit diffraction gratings, wherein numbers from first to Jth are assigned to said unit diffraction gratings sequentially from one end of said optical waveguide to the other end thereof; and the reflectance Ri from the ith (2≦i≦J) unit diffraction grating is given by reflectance Ri=Ri−1/(1−Ri−1)2 (3).
 15. An optical code division multiplexing transmission method, comprising: an encoding step of encoding an optical pulse signal by using optical phase code to generate said optical pulse signal as an encoded optical pulse signal; and a decoding step of decoding said encoded optical pulse signal by using said same code as said optical phase code to generate an autocorrelation waveform of the optical pulse signal, wherein said encoding step and said decoding step are executed by using the optical pulse time spreader according to claim
 2. 16. An optical code division multiplexing transmission method, comprising: an encoding step of encoding an optical pulse signal by using optical phase code to generate said optical pulse signal as an encoded optical pulse signal; and a decoding step of decoding said encoded optical pulse signal by using said same code as said optical phase code to generate an autocorrelation waveform of the optical pulse signal, wherein said encoding step and said decoding step are executed by using the optical pulse time spreader according to claim
 3. 17. An optical code division multiplexing transmission method, comprising: an encoding step of encoding an optical pulse signal by using optical phase code to generate said optical pulse signal as an encoded optical pulse signal; and a decoding step of decoding said encoded optical pulse signal by using said same code as said optical phase code to generate an autocorrelation waveform of the optical pulse signal, wherein said encoding step and said decoding step are executed by using the optical pulse time spreader according to claim
 4. 18. An optical code division multiplexing transmission device, comprising: an encoder that encodes an optical pulse signal by using optical phase code to generate said optical pulse signal as an encoded optical pulse signal; and a decoder that decodes said encoded optical pulse signal by using said same code as the optical phase code to generate an autocorrelation waveform of said optical pulse signal, wherein said encoder and said decoder are optical pulse time spreaders according to claim
 2. 19. An optical code division multiplexing transmission device, comprising: an encoder that encodes an optical pulse signal by using optical phase code to generate said optical pulse signal as an encoded optical pulse signal; and a decoder that decodes said encoded optical pulse signal by using said same code as the optical phase code to generate an autocorrelation waveform of said optical pulse signal, wherein said encoder and said decoder are optical pulse time spreaders according to claim
 3. 20. An optical code division multiplexing transmission device, comprising: an encoder that encodes an optical pulse signal by using optical phase code to generate said optical pulse signal as an encoded optical pulse signal; and a decoder that decodes said encoded optical pulse signal by using said same code as the optical phase code to generate an autocorrelation waveform of said optical pulse signal, wherein said encoder and said decoder are optical pulse time spreaders according to claim
 4. 